1. Field of the Invention
This invention relates to a vibration generator, designed more particularly for energizing vibrating apparatus of the type employed in handling circuits and treatment stations for bulk and loose materials or products, such as vibrating feeders or conveyors, feeders, screens, sieves, etc. It is known that when a vibratory motion is impressed on a resilient mass, the latter vibrates at an excitation frequency fe with a predetermined amplitude a depending on the applied torque in the case of an out-of-balance excitation system.
The apparatus assumes a resonance condition when the ratio: fe/fp = 1, wherein fp is the inherent frequency of the resilient mass.
Under these conditions, three states have to be taken into account:
I. below the resonance state, we have: fe/fp &lt; 1 PA1 Ii. at the resonance state, we have: fe/fp = 1 PA1 Iii. above the resonance state, we have: fe/fp &gt; 1
2. Description of the Prior Art
This led manufacturers to propose three specific groups of vibration generators. A first type comprising a connecting-rod and crankshaft system operates well below resonance level and provides wide amplitudes at low frequency values; it is utilized chiefly for transporting products of low apparent density. Another type operates close to resonance frequency, either slightly below (sub-critical regime) or slightly above (super-critical regime) this frequency. Under sub-critical regime the vibrated resilient mass admits a variable load on the transporting trough, shoot or chute without entailing a substantial amplitude modification. On the other hand, under super-critical regime a variation in the trough load is attended by a reduction in the vibration amplitude and therefore in the apparatus output. This second type of apparatus comprises both eccentric-operated and electromagnet-operated vibration generators. In electromagnet-operated vibration generators the power consumption is relatively low since it takes advantage of the resonance power. More particularly, the electromagnet excitation system is particularly adapted for adjusting the vibration amplitude during the operation of the apparatus or in other words, for adjusting the apparatus output, but if large outputs are contemplated the generator may become unduly cumbersome for, since the rate of propagation of the material in the chute is inversely proportional to the chute vibration frequency, it would be necessary for obtaining relatively high outputs to use a frequency of less than 50 Hertz, which is obviously scarcely feasible under normal service conditions. Finally, vibration generators of the third type operate well beyond resonance level and have therefore a stable output attended however by the inconvenience of requiring an excitation power consumption of two to three times the energy necessary for maintaining the motion due to the necessity of passing rapidly through the resonance zone. In general, to vary the output, one is led to stop the vibration generator. Now, in certain cases it may be very useful to control this change during the operation of the apparatus by using manual or automatic servo means for varying the output as a function of a variable factor from an apparatus located downstream. To vary an output during operation, various factors or parameters may be controlled, such as the frequency itself (damping the resilient mass), the excitation frequency (notably in the case of out-of-balance excitation systems), but due to their position beyond the resonance frequency, only one fraction of the range of output adjustments can be obtained. Apparently, the most advantageous method consists in varying the amplitude during the operation of the apparatus, so that a range from zero to maximum output can be attained continuously. This last-mentioned solution was actually choosen by the Applicant, considering the fact that the first two solutions led to scarcely reliable and rather inaccurate constructions. Means for varying the amplitude of vibration during service have already been proposed, notably for out-of-balance generators, but this construction is particularly complicated from the mechanical standpoint.